As group III nitride semiconductors have a direct energy transition band gap equivalent to the region from visible light to ultraviolet light, enabling highly efficient emission of light, they have been commercialized as light-emitting diodes (LEDs) and laser diodes (LDs). In addition, they also have a potential for use as electronic devices capable of providing characteristics which cannot be obtained with conventional group III-V compound semiconductors, such as the appearance of a two-dimensional electron layer, due to the characteristic piezoelectric effects in group III nitride semiconductors, at the heterojunction interface between aluminum gallium nitride (AlGaN) and gallium nitride (GaN).
As group III nitride semiconductors have a dissociation pressure which reaches 2,000 atmospheres at the growth temperature of single crystals, it is difficult to grow single crystals, and the use of single crystals of group III nitride semiconductors as substrates used for epitaxial growth in the manner of other group III-V compound semiconductors is difficult under the present circumstances. Therefore, substrates composed of dissimilar materials such as sapphire (α-Al2O3) single crystals and silicon carbide (SiC) single crystals are used as substrates for epitaxial growth.
Large lattice mismatch is present between these dissimilar substrates and the group III nitride semiconductor crystals epitaxially grown thereon. For example, a lattice mismatch of 16% is present between C-plane sapphire and gallium nitride (GaN), while a lattice mismatch of 6% is present between SiC and gallium nitride. In general, in the case of the existence of a high degree of lattice mismatch such as this, it is difficult to grow crystals directly on a substrate by epitaxial growth and, even if the crystals are grown, crystals with satisfactory crystallinity are not obtained. Therefore, as indicated in Japanese Patent No. 3026087 or Japanese Unexamined Patent Publication No. H4-297023, in the case of epitaxial growth of group III nitride semiconductor crystals on a sapphire single crystal substrate or SiC single crystal substrate using metal organic chemical vapor deposition (MOCVD), a method is typically carried out in which a layer referred to as a low-temperature buffer layer composed of aluminum nitride (AlN) or AlGaN is first deposited on the substrate, followed by epitaxial growth of group III nitride semiconductor crystals thereon at a high temperature.
The use of this buffer layer technology made it possible to produce high-quality group III nitride semiconductor crystals, enabling practical application of a blue-green LED and a violet LD.
In addition, attempts were also made without using the aforementioned low-temperature buffer layer technology (see, for example, International Publication WO 02/17369 and Japanese Unexamined Patent Publication No. 2003-243302). The use of this technology made it possible to obtain highly efficient blue LEDs comparable to low-temperature buffer layer LEDs.
The GaInN emission layer used in blue LEDs has lattice mismatch with respect to GaN and, for example, an extremely large piezoelectric field of about 1 MV/cm has been reported to be normally generated in a Ga0.9In0.1N layer which is grown on the (0001) plane of GaN crystals (see, for example, T. Takeuchi et al., Jpn. J. Phys., Vol. 36 (1997) pp. L382-L385).
In general, if an electric field is present in a quantum well layer, as the energy band of the quantum well layer shifts considerably as the electric field increases, the shapes of the wave functions of electrons and holes deviate mutually and differently, and the integral where both wave functions overlap becomes smaller. In other words, a significant change in optical characteristics occur in the form of a decrease in emission efficiency or absorption efficiency (quantum confinement Stark effect) (see D. A. B. Miller et al., Phys. Rev. Lett., 53 (1984) 2173). Deterioration of device characteristics occurs resulting in problems in devices, such as light-emitting devices, due to this type of piezoelectric field.
In contrast to the external quantum efficiency of group III nitride semiconductor LEDs being 43% for violet and 40% for blue, it is only about 15% for green, thus resulting in a desire to increase the efficiency of green LEDs at long wavelengths. Factors behind the low external quantum efficiency of green LEDs consist of not only poor crystallinity of the emission layer GaInN (crystal deterioration caused by increased In composition), but also the afore-mentioned effects produced by piezoelectric fields due to the use of longer wavelengths.
A method for controlling the piezoelectric field according to the orientation of the growth plane is known as a measure for reducing the piezoelectric field (see, for example, Japanese Unexamined Publication No. H11-112029). According to this method, inclination of the growth orientation reduces the piezoelectric field, and examples of planes in which a piezoelectric field does not occur include the A-plane (11-20) and the M-plane (10-10). A known example of a method for obtaining A-plane (11-20) epitaxial growth involves the application of R-plane sapphire for the substrate (see, for example, Lei et al., J. Appl. Phys., Vol. 80 (1993), p. 4430).
Among sapphire substrates, R-plane substrate has the lowest production cost, and is currently available in sizes of about 8 inches. On the other hand, as routinely used C-plane sapphire substrate is the most difficult type of sapphire substrate to produce while also being expensive, LEDs produced on this substrate are expensive.
Thus, being able to produce LEDs using R-plane sapphire substrates would make it possible to increase the efficiency and reduce the production costs of long-wavelength green LEDs, and is therefore considered to be extremely useful. However, group III nitride semiconductors produced on R-plane sapphire substrates are susceptible to the formation of pits in the surface, and it has been difficult to produce a high-quality group III nitride semiconductor layer on an R-plane sapphire substrate.
Attempts have been made, during GaN growth on an R-plane sapphire substrate, to improve the surface flatness by increasing the thickness of the GaN layer to about 75 μm using growth technologies such as epitaxial lateral overgrowth (see, for example, C. Q. Chen et al., Jpn. J. Appl. Phys., Vol. 42 (2003), p. L640). In the case of epitaxial lateral overgrowth, the process is complex due to the need to form a mask pattern and to repeat the growth several times by re-growing. In addition, a thick film group III nitride semiconductor layer is required, thereby increasing costs.
In addition, an attempt has also been made to introduce a zinc oxide layer between an R-plane sapphire substrate and a group III nitride semiconductor layer (see, for example, Japanese Unexamined Patent Publication No. H7-240374). In this method, as the zinc oxide layer is formed by sputtering, several steps of growth are required.